The present invention relates to an electrophotosensitive material used in an image forming apparatus such as an electrophotographic copying apparatus or the like.
As the electrophotosensitive material, there is recently used a function-separated type electrophotosensitive material in which the charge generating function and the charge transferring function are respectively achieved, as separated from each other, by a charge generating material for electric charge generating with exposure to light and a charge transferring material for transferring a generated charge. In such function-separated type electrophotosensitive material, it is easy to enhance the charge generating function to improve the sensitivity.
As examples of the function-separated type electrophotosensitive material above-mentioned, there are available (i) a multilayer type photosensitive layer unit having a charge generating layer containing a charge generating material and a charge transferring layer containing a charge transferring material, and (ii) a single-layer type photosensitive layer containing both a charge generating material and a charge transferring material.
Examples of the function-separated type electrophotosensitive material above-mentioned, include (i) an organic electrophotosensitive material using, as a photosensitive layer of a multilayer type or single-layer type photosensitive layer unit, an organic layer containing, in binding resin, functional components such as a charge generating material, a charge transferring material and the like, and (ii) a composite-type electrophotosensitive material in which the organic layer above-mentioned, a semiconductor thin film and the like are combined to form a multilayer type photosensitive layer unit. The electrophotosensitive materials above-mentioned are suitably used since they have a variety of choices for materials to be used and present good productivity and high degree of freedom for function designing.
However, there is the likelihood that the organic photosensitive layer in the organic electrophotosensitive material or composite-type electrophotosensitive material is decreased in charge amount, sensitivity and the like when an image forming process of charging, light exposure, charge eliminating and the like is repeated.
To prevent such a decrease in charge amount, sensitivity and the like, there have been proposed (i) an electrophotosensitive material using, in addition to a normal charge transferring material, another charge transferring material of an m-phenylenediamine compound excellent in properties for preventing a decrease in charge amount, sensitivity and the like, and (ii) an electrophotosensitive material using the m-phenylenediamine compound above-mentioned together with a perylene compound (a charge generating material) also excellent in properties for preventing a decrease in charge amount, sensitivity and the like.
However, the organic electrophotosensitive material or composite-type electrophotosensitive material containing the m-phenylenediamine compound and the like presents the problem of sudden decrease in sensitivity when the electrophotosensitive material is irradiated by light from a fluorescent lamp, a halogen lamp, a xenon lamp, the sun or the like, particularly at the time when the electrophotosensitive material is heated, for example, during the operation of the image forming apparatus.
Such a decrease in charge amount and sensitivity due to repeated light exposures or such a sudden decrease in sensitivity due to light irradiation is considered to be caused by the fact that the charge transferring material absorbs visible light or ultraviolet rays contained in the irradiated light, causing the charge transferring material to be excited, or that the charge transferring material is excited by an energy transmitted from other light absorbing substance such as the charge generating material or the like. This produces a dimerization or decomposition reaction, causing the charge transferring material to be changed to a substance acting as a carrier trap to decrease the sensitivity of the electrophotosensitive material.